Electroacoustical transducer



y 1950 M. 1.. GAYFORD 2,509,224

ELECTROACOUSTICAL TRANSDUCER Filed Sept. 16, 1944 3 Sheets-Sheet 1 IO 25I9 24 I3 I ventor NuunELLnwREncEQRX iimkv 7 By Altorhey May 30, 1950ELECTROACOUSTICAL TRANSDUCER Filed Sept. 16, 1944 M. L. GAYFORD F/GB.

3 Sheets-Sheet 2 Inventor M \umEL Lnwamcx. Gavin?) y [tome y 1 y 1950 M.1.. GAYFORD 2,509,224

ELECTROACOUSTICAL TRANSDUCER Filed Sept. 16, 1944 3 Sheets-Sheet 3 50007000 IOOOO FIGS.

Q I n mentor m hcnnELLeunmcE M WW A Home Patented May 30, 1950 UNITEDSTATES TENT OFFICE ELECTROACOUSTICAL TRANSDUCER Application September16, 1944, Serial No. 554,407

In Great Britain September 28, 1943 2 Claims.

The present invention relates to electro-acoustical transducers of theelectro-magnetic moving coil type which may be either microphones orreceivers.

The invention will be presently explained in terms of two embodimentsillustrated in the accompanying drawings inwhich:

Fig. 1 shows a side elevation of a moving coil microphone sectionedalong the lines BB of Fig. 2;

Fig. 2 shows a top view sectioned along the lines A-A of Fig. 1;

Fig. 3 shows an enlarged view of the diaphragm and magnetic circuit ofthe microphone to show the details more clearly;

Fig. 4 shows a plan view of the magnet employed in the microphone;

Fig. 5 shows a sectional view of another type of microphone according toth invention; and.

Fig. 6 shows output response curves for moving coil microphones in whichthe abscissaeare the frequencies in period per second (pzs) and theordinates are output levels in decibels referred to an arbitrary zerolevel. Curve A is for a microphone according to the invention, and curveB is for a microphone of the unimproved type to show the nature of theirregularity corrected by the invention.

In order that the significance of the invention may be clearlyappreciated, an account of the present state of the art will first begiven.

A considerable amount of attention has been paid in recent years to thedesign of moving coil microphones and receivers in order to improvetheir fidelity of response over a wide frequency range. Instruments ofthis type are described in the following two articles in the Bell SystemTechnical Journal:

(it) Oct. 1931, page 565, "Moving coil telephone receivers andmicrophones by E. C. Wente and A. L. Thuras.

(1)) July, 1936, page405, A non-directional microphone by R. N. Marshalland F. F. Romanov.

and also in the following article in the Journal of the Society ofMotion Picture Engineers of the U. S. A.:

(0) Dec. 1931, page 9'77, A moving cOil microphone for high qualitysound reproduction by W. C. Jones and L. W. Giles.

These instruments gave fairly good results, and employ a now common formof diaphragm of aluminium alloy consisting of a domed central portionwith a flat annular surrounding portion so designed that the dome movessubstantially as a piston. The diaphragm carries alight cylindrical coilwhich moves in an annular magnetic field created between a central domedpole piece and a :cc-axial outer pole-piece. The space between thediaphragm and the central domed pole-piece.

forms a chamber to which is connected a narrow passage formed by thecoil slot. The arrange ment constitutes efiectively a Helmholtzresonator of which the coil slot and connecting passages constitute theneck. The constants of this resonator have to be carefully chosen sothat a suitable resonance frequency is obtained,'and appropriateacoustic resistance is introduced in the neck to damp the resonance.This acoustic resistance is obtained by connecting the. coil slot to therelatively large hollow space inside the case of the instrument by verynarrow passages formed either by slits in metal parts or in the meshesof fabrics such as silk. British Patent specification No. 333,579 forexample, describes one method of obtaining this acoustic resistance, andvarious other Ways of achieving th same result have been since proposed.It may be now regarded as a quite usual practice to provide some form ofacoustic resistance connection between the coil slot and the maininstrument case.

Instruments designed along these lines have generally been found to havean undesirable irregularity in the response characteristics, in themiddle and upper part of the useful frequency range, which isattributable to the fact that the resonance frequency of the chamberbehind the diaphragm is too low, and it has not been found practicableto raise it sufficiently with the already known designs. Curve 3 of Fig.6 illustrates the nature of this irregularity, and may be compared withcurve A which is for an instrument corrected according to the invention.

The principal object of the present invention is to remedy the dimcultybyreducing the effective mass associated with the neck of the Helmholtzresonator behind the diaphragm so that the resonance frequency may beraised by the desired amount. This is done by providing an additionalacoustic resistance connection between the chamber behind the diaphragmand the relatively large space inside the case of the instrument bymeans of a passage or passages through the central pole-piece. Thisprovides an additional neck to the Helmholtz resonator with acorresponding effective mass in shunt with the mass provided by thepreviously mentioned coil slot connection. This will be explained morefully later on.

The relatively large space inside the case has a negligible stiffnesseffect except at the lowest audio frequencies (for example, below about150 pzs). The frequencies which are of interest in the presentinvention, as already explained, are the middle and upper frequencies ofthe range. When therefore, it is stated that the space inside the casehas a negligible stiffness effect, the qualification just explained issupposed to be understood, and is immaterial in the circumstances of theinvention.

Attention is directed to the arrangement shown in Fig. 9 of the articleby Wente and Thuras and also in the article by Jones and Giles, anddescribed also in British patent specification No. 382,668. This figureshoWs a small resonant chamber in the dome of the central pole-piececonnected to the chamber behind the diaphragm by a narrow slit orpassage. The purpose of this small chamber is to correct the responsecharacteristic of the instrument for an unexpected irregularity, shownin curve A of Fig. 3

of the article by Jones and Giles, the reason for V which was not veryclearly understood, but which is now believed to be due to a resonancecaused by the diaphragm breaking up and oscillating in higher modes thanthe fundamental so that its central domed portion ceases to vibratesubstantially as a piston. This particular effect is a sharpirregularity at the upper end of the frequency range and is easilydistinguished from the much smoother and more extensive irregularity,shown in curve B of the accompanying Fig. 6, which is caused by the lowresonance frequency of the chamber behind the diaphragm. This sharpirregularity can be avoided by proper care in the design and manufactureof the diaphragm so that the small chamber is not required, and it isnot employed in the embodiments of the present invention.

Reference has been made to this arrangement because of its apparentsimilarity to the principal feature of the present invention, in orderthat the distinction may be clearly appreciated. The present inventionis concerned with means for raising the resonance frequency of thechamber behind the diaphragm and not with difiiculties associated withimperfect performance of the diaphragm, which are better dealt with bypaying careful attention to its design and. construction.

According to the invention, therefore, there is provided anelectro-acoustio transducer comprising a case having therein a hollowspace of negligible stiffness effect, a diaphragm carrying a coiladapted to move in a slot across which is established a magnetic field,a substantially closed resonating chamber behind the diaphragm, and anacoustic resistance connection between the chamber and the hollow spacethrough a path which does not include the slot.

The invention also provides an electro-acoustic transducer comprising acase having therein a hollow space of negligible stiffness effect, amagnet having a central pole-piece and an annular pole-piece, the twopole-pieces being separated by a narrow slot, a diaphragm carrying acircular coil adapted to move in the slot, a substantially closedresonating chamber behind the diaphragm, and an acoustic resistanceconnection through the central pole-piece between the chambar and thehollow space.

According to another aspect, the invention consists in anelectro-acoustic transducer comprising a diaphragm carrying a coiladapted to move in a narrow slot, magnetic circuit means forestablishing a magnetic field across the slot, a substantially closedresonating chamber behind the diaphragm communicating with the slot, anenclosing case having therein a hollow space of negligible stiffnesseffect, an acoustic resistance connection between the slot and thehollow space, and a further direct acoustic resistance connectionbetween the chamber and the hollow space.

The two embodiments of the invention will now be described. Althoughboth the embodiments are microphones, the same principles andconstructions are applicable to moving coil receivers, which differ frommicrophones only in so far as they must be designed for handling higherpowers, and usually also to work into a different acoustic impedance.

Figs. 1, 2 and 3 show an electromagnetic microphone of the moving coiltype which is generally similar to that illustrated in the article byMarshall and Romanov. It has spherical case I in the upper part of whichthere is mounted a circular magnet 2 having a central pole 3 and asurrounding annular pole A top View of the magnet is shown in Fig. 4 andindicates that the annular pole is interrupted by four slots 5 providingcommunication between the annular gap 6 and the interior of the maincase I of the microphone. Fixed to the central pole 3 is a circular discl forming one pole-piece, and fixed to the annular pole t is acorresponding annular pole-piece 8. Between the two pole-pieces is thecoil slot 9. These details are most clearly seen in Fig. 2.

The diaphragm is of well known type, and comprises a central domedportion H), a substantially flat periphery H and an intermediatecorrugated annular portion l2. It is clamped between the pole-piece 8and an annular shoulder 13 on the inside of the case I (see Fig. 1). Thecase in the neighbourhood of this shoulder is fragmentarily shown inFig. 2. The moving coil [4 is fixed to the diaphragm and is centrallylocated in the coil slot 9.

A domed plate I5 is fixed on the central polepiece 1 just below thedomed portion of the diaphragm. This plate is perforated with a numberof holes l6, and a disc l? of silk or gauze covers the under surface ofthe plate. A cylindrical channel l8 passes right through the centre ofthe pole-piece l and magnet 2, and forms a wide communication betweenthe space [9 under the plate l5 and the inside of the case I.

The coil slot 9 communicates with an annular space 23 closed by anannular plate 2i perforated by a ring of holes 22, the upper side ofwhich is covered by an annular sheet of silk or gauze. This provides theusual acoustic resistance connection between the coil slot 9 and theinside of the case through the gap 6 of the magnet, which, as alreadystated, communicates directly therewith through the slots 5.

The magnetic circuit is bolted to a plate 33 by four screws 32 (of whichonly one is shown in Figs. 1 and 2) located in the slots 5 of the magnet2.

By means of the perforated plate l5 and the channel it, a connection isestablished between the chamber behind the diaphragm and the inside ofthe case in addition to the connection through the coil slot. Thisadditional connection illustrates the principal feature of theinvention. Preferably the channel is is stuffed with lambs wool or thelike to damp out any propagation effects in the channel.

As an alternative to the perforated plate [5, two such plates fittingclosely together one in-.

side the other with the holes registering, may be employed, with thesilk sandwiched between them. This provides a possibly better method offixing the silk which may not stick very satisfactorily to a curvedsurface.

In accordance with Fig. 4 of the article by Wente and Thuras quotedabove, let S1 be the stiffness of the air in the chamber behind thediaphragm, and let M1 and R1 be the total eifective mass and acousticresistance, respectively, of all the narrow passages which constitutethe neck of the Helmholtz resonator. Then it is well known that theresonance frequency is given by:

It has already been stated that in the microphones or receivers of thetype under considera tion it has been found to be impracticable to raisethe value of F insufficiently high. This is principally because R1 andM1 are not independent, and by means of the coil slot path alone it hasnot generally been found possible to obtain a suitable value of R1without associating therewith a value of M1 which is too large. By meansof the extra connection according to the invention a further mass isprovided which is effectively in parallel with the mass associated withthe coil slot connection, and thereby M1 is considerably reduced. It is,of course, understood that R1 will also be changed, but by adjusting theacoustic resistance associated with the coil slot in known way it hasbeen found that a suificiently low value of M1 can be obtained accordingto the invention with a suitable value of R1 at the same time.

The remaining features of the microphone shown in Fig. 1 are well knownand for the most part are described in the quoted articles. Thus 23 isthe base equalising tube which forms an appropriate connection betweenthe inside and outside of the case. 24 is the front grille covered withsilk for providing appropriate acoustic resistance for damping theresonance of the chamber in front of the diaphragm, according to usualpractice. spherical in form in order to obtain non-directionalproperties in accordance with the principles explained in the article byMarshall and Romanov. The polar equalising screen is shown at 25.

Fig. 5 shows another embodiment of the invention comprising a movingcoil microphone intended for a special purpose where a much smallerfrequency range had to be covered. In this case it was found preferableto close up the coil slot entirely so that there was no communicationwith the inside of the case, and to provide all of the mass M1 andacoustic resistance R1 by means of a connection through the centre ofthe magnet in much the same way as in the embodiment previouslydescribed. In this way the desired amount of acoustic resistance isobtained with a smaller value of M1 than by means of the coil slotconnection.

In Fig. 5 the magnet N12 is a plain hollow cylinder, the centralpole-piece IQ! being a smaller coaxial cylinder with a flange at thelower end making contact with the magnet. The annular pole-piece Hi8surrounds the upper end of the pole-piece IEI'I, the coil slot IllBbeing formed between them. The magnet and polepieces are fixed togetheras a unit by screws I26, and the space I2! between is filled by means ofa solidifying compound such as styrene, so as to seal off the lower endof the coil slot.

The case of the microphone is made The domed dia hragm I28 isofsimilarl-type to that shown-inFig. 2 and isclampedbetWEfin thepolerpiece H38 and the perforatedfrontplate 1 I293. The whole is securedinside a cylindrical case it: spun over a closing plate I30.

The relatively largehollow space I3I' insided the pole-piece I0! isclosed at the upper endbyr a flat perforated plate H5 covered with asheet.

of silk Hi. There is then formed behind the. diaphragm a resonatingchamber I34 which .is much larger than the corresponding chamber in thepreviously described embodiment, since there.

is no dome mounted on the pole-piece IN. A

larger chamber is required in this case since the resonance frequencydesired is much lower.

The silk covered perforations in the plate H5 provide the neck for theHelmholtz resonator, and introduce the desired mass and acousticresistance.

The hollow space I3! may be stuffed with lambs wool or the like ifnecessary in order to avoid propagation effects.

In either of the embodiments which have been described, it is preferableto use for the magnet a material which has been heat treated and/orcooled in a magnetic field to predetermlne the direction in which it isto be subsequently magnetised. This increases the efiiciency of the manetic material and enables the maximum flux to be obtained in the coilslot. The use of such a magnet accounts for the fact that in Fig. 6,curve A is at a higher level than curve B.

It will be seen that the main feature of the invention whereby a centralconnection is established between the chamber behind the diaphragm andthe inside of the case may be employed whether or not a connection isalso provided between the coil slot and the interior of the case. It isto be noted that this central connection has another advantageparticularly applying to the embodiment described with reference toFigs. 1 to 4.. Owing to the fact that the acoustic resistance isintroduced very close to the centre of the diaphragm by the domed platemounted on the central pole-piece, any tendency for the dome to break upand to oscillate in a higher mode through lack of rigidity of thediaphragm dome is largely prevented due to the local damping of thecorresponding resonances by this acoustic resistance. The action issimilar to the damping action of the backing plate holes commonly provided close behind the diaphragm of a condenser microphone.

What is claimed is:

1. An electro-acoustic transducer including a diaphragm, a coil carriedthereby, magnetic circuit means for establishing a magnetic field acrossa space having the form of a slot, said coil being positioned to move insaid slot, a substantially closed resonating chamber positioned behindsaid diaphragm communicating with said slot, a case enclosing a hollowspace of negligible stiffness effect, an acoustic resistance connectionpassing between said slot and said hollow space independently of thechamber, a further direct acoustic resistance connection passing betweensaid chamber and said hollow space, and said magnetic circuit meanscomprisin a central pole-piece and an annular pole-piece spacedtherefrom by said slot, and a domed acoustic resistance plate mounted onsaid central pole-piece so as to form part of the boundary of saidchamber behind said diaphragm.

2. An electro-acoustic transducer including a case inclosing arelatively enlarged hollow space to be of negligible stiffness efiect, amagnet lo cated in said case and having a, central pole piece and anannular pole piece, said two polepieces being separated by a narrow gap,a diaphragm carrying a coil positioned to move in said gap, asubstantially closed resonating chamber positioned behind saiddiaphragm, said central pole piece being formed to establish an acousticresistance connection between said chamber and said hollow spaceindependently of the gap and said annular pole piece being interruptedby slots to provide communication between said annular gap and saidhollow space independently of said chamber.

MICHAEL LAWRENCE GAYFORD.

8 REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,952,357Blumlein et a1. Mar. 27, 1934 1,954,966 Thuras Apr. 17, 1934 1,964,606Thuras June 26, 1934 2,084,944 Cornwell June 22, 1937 2,095,373 SpeerOct. 12, 1937

